Numerical modeling flow and heat transfer in dimpled cooling channels with secondary hemispherical protrusions

Flow characteristics and heat transfer performances of rectangular dimpled channels with secondary protrusions are numerically analyzed and compared in this research work. The special arrangement is a conventional dimpled cavity with a small-size secondary protrusion placed upstream of the dimple. The objective is to determine the most optimal configuration for augmenting heat transfer rates with minimized pressure drop penalties, and providing flow details of secondary protrusions with different arrangements or geometries. The main considered parameters are the height ratios and relative location of the secondary protrusions placed upstream of the dimple. All turbulent fluid flows and surface heat transfer results are obtained using computation fluid dynamics with a validated k-ε RNG (Re-Normalization Group) turbulence model in eight rectangular channels with different secondary protrusion arrangements. From this investigation, it is found that secondary protrusions cause downward flows and reduce the extent of the recirculating flows in the adjacent primary dimple and then greatly improve the averaged local heat transfer of the corresponding primary dimple surface. In addition, secondary protrusions can improve the reattachment and strengthen the shedded vortex structures from the dimple. The heat transfer is enhanced with increasing height of the secondary protrusions but the pressure drop is also increased significantly. The largest Nu/Nu0/(f/f0) factor is obtained by a depth ratio of 0.1 and the largest Nu/Nu0/(f/f0)1/3 factor is provided by a depth ratio of 0.2. The vortex structure formed downstream of the secondary protrusions remains to take effect and develop in the primary dimple if the secondary protrusions are located near the centerline area as in Case C1. The acting vortex structures can improve the spanwise flow and improve the turbulent mixing downstream of the dimple. With a secondary protrusion located away from the centerline, the heat transfer enhancement is weakened. The heat transfer augmentation and overall thermal performance advantages of a dimpled channel with secondary protrusions are evident compared with conventional dimpled channels.